This invention relates to semiconductors and coatings of semiconductive materials on electrical insulator structures made of alumina. More particularly, the invention relates to electrical discharge producing devices having a pair of electrodes which are connected by an electrical insulator structure coated with a semiconductive material having an electrical resistance that increases as a function of the distance inwardly from the exposed surface of the semiconductive material.
While the present invention will have utility in all types of applications where semiconductors are used, it has particular advantages in the field of spark plugs and jet engine igniters. Thus, it is in this connection that the invention will be described.
A spark plug or jet engine igniter of the type with which the invention is concerned generally comprises a tubular shell, and a centrally located electrode separated therefrom by an annular insulator. A portion of the center electrode at one end of the insulator is in spark gap relation with a portion of the shell or an associate ground electrode.
In a low voltage igniter an electrically semiconductive material is provided, electrically connecting the center and ground electrodes.
In one proposed construction, the tubular shell has been provided with a radially inwardly extending shoulder, and the center electrode has been provided with a radially outwardly extending shoulder, and an angular disc of semiconductive material having appreciable thickness has been positioned between the shell and center electrode in such manner that the two electrodes contact the semiconductive material only on its exposed surface. It is suggested that by limiting the contact to the exposed surface of the semiconductive material a spark of maximum intensity can be achieved adjacent the exposed surface. However, a considerable amount of electricity flows through the interior of the conductive material where its flow does not contribute to the spark intensity. A further difficulty exists in the above-described design in that the sides of the disc of semiconductive material must be insulated from the center electrode and shell, so that a partial short circuit between the electrodes is not provided by the portions of the semiconductive material inwardly from its exposed surface.
In another type of structure which has been proposed heretofore, a thin engobe coating of a semiconductive material having a relatively high conductivity is provided on the surface of a fixed insulator positioned between the electrodes. One difficulty inherent in the use of thin coatings of semiconductors having relatively high conductivity is that the service life of these thin coatings is relatively short due to arosion caused by the spark discharge and highly turbulent gases.